1. Field of the Invention
The present invention generally relates to a folded cascode amplifier, and more particularly to a folded cascode amplifier with an enhanced slew rate.
2. Description of Related Art
A folded cascode amplifier is one of operational transconductance amplifiers (OTA). The folded cascode amplifier has a large gain and a large signal swing, and is commonly made in a complementary metal oxide semiconductor (CMOS) integrated circuit to be a basic building block for a variety of applications.
FIG. 1A shows a circuit diagram of a conventional folded cascode amplifier, which includes a (p-type) input pair 11 made of p-type metal oxide semiconductor (PMOS) transistors M1 and M2. The slew rate (SR) of the conventional folded cascode amplifier as shown in FIG. 1A so is low such that the amplifier possesses nonlinearity characteristics. Analytically speaking, the transistor M1 is turned off and the transistor M2 is turned on when a non-inverting input voltage Vin+ is near a power VDD and an inverting input voltage Vin− is near a ground GND. As a result, an entire current 2Ib flows through the transistors M2 and M4. As transistors M3 and M4 form a current mirror, a current of the transistor M3 is also 2Ib. As the transistor M1 is off, the current of the transistor M3 is provided from a path made of transistors M9, M7 and M5. Moreover, as the transistors M9 and M10 form a current mirror, a current of the transistor M10 is also 2Ib, which charges a load capacitance C at an output node Vout via a transistor M8, therefore resulting in a slew rate SR=2Ib/C. An input range of the folded cascode amplifier as shown in FIG. 1A is between 0 and VDD−Vds_M0−Vt_M1.
FIG. 1B shows a circuit diagram of another conventional folded cascode amplifier, which includes an (n-type) input pair 12 made of n-type metal oxide semiconductor (NMOS) transistors Q1 and Q2. The folded cascode amplifier as shown in FIG. 1B may charge a load capacitance CL at an output node Vo, therefore resulting in a slew rate similar to that of FIG. 1A, and an input range between Vss+Vds+Vt and VDD.
FIG. 2 shows a circuit diagram of a further conventional folded cascode amplifier, which includes an n-type input pair 21 made of NMOS transistors Q1/Q2 and a p-type input pair 22 made of PMOS transistors Q3/Q4. The input range of the folded cascode amplifier as shown in FIG. 2 is between 0 and VDD, which is wider than those of FIGS. 1A and 1B. However, a slew rate of FIG. 2 is the same as that of FIGS. 1A and 1B, that is, SR=2Ib/C.
FIG. 3 shows a circuit diagram of a further conventional folded cascode amplifier, which has a larger transconductance, a larger gain and a larger slew rate than the folded cascode amplifiers discussed above. Analytically speaking, transistors M1a and M1b are turned off and a transistor M2a enters into a deep triode region when a non-inverting input voltage Vin+ is near a power VDD and an inverting input voltage Vin− is near a ground GND. As a result, an entire current 2Ib flows through transistors M2b, M11 and M3b. As transistors M3a and M3b form a current mirror and their size ratio is M3a:M3b=K:1, a current of the transistor M3a is thus 2KIb, which is provided from a path made of transistors M9, M7 and M5. Moreover, as the transistors M9 and M10 form a current mirror, a current of the transistor M10 is also 2KIb, which charges a load capacitance C at an output node Vout via a transistor M8, therefore resulting in a slew rate SR=2KIb/C.
FIG. 4 shows a modified folded cascode amplifier of FIG. 3. In addition to a p-type input pair 41 made of PMOS transistors, an n-type input pair 42 made of NMOS transistors is further used. The input range of the folded cascode amplifier as shown in FIG. 4 is between 0 and VDD, which is wider than that of FIG. 3. However, a slew rate of FIG. 4 is the same as that of FIG. 3, that is, SR=2KIb/C.
For the reason that the slew rates of the conventional folded cascode amplifiers are not large enough, a need has thus arisen to propose a novel folded cascode amplifier with an enhanced slew rate in order to enlarge a range of applications.